A toner-transfer printing device such as a laser printer creates a printed image by printing an array of picture elements, referred to as pixels, on the print medium. Any image, whether text or graphics, to be printed by the printing device is represented in the form of a set of discrete pixels. Images such as photographs are, however, continuous analog representations of objects in the real world. Other types of images, including text and computer graphics images, may also contain representations of analog forms. In all cases, it is necessary to represent the desired image with discrete pixels if the image is to be printed by a toner-transfer printing device.
One common representation is a raster or orthogonal grid of pixels. A rasterized image typically contains a large number of discrete pixels, each of which has dimensions such as location coordinates and intensity. The area of the image represented by a pixel is referred to as a pixel cell. The intensity dimension for black and white images is a value indicating a shade of gray. The intensity dimension for a color image is a set of values indicating levels for a set of colors such as cyan, magenta, yellow and black. This pixel feature for both black and white as well as color images will hereinafter be referred to as the pixel's “gray level”.
A rasterized representation of an original image can visibly distort the image when the spatial frequency of the raster is less than the highest discernable frequencies present in the image. Straight lines, sharp edges and corners and other high contrast features contain very high frequencies and often cannot be rendered accurately. If a high-frequency feature such as a sharp edge is rendered with a pixel resolution that is insufficient to capture all of the visible information contained within the image, artifacts will appear. This phenomenon is known as aliasing. Certain visual artifacts are particularly problematic: diagonal lines and boundaries between different regions tend to have a jagged step or staircase appearance that is often quite visible. The appearance of jagged edges and other artifacts in digitized images is a significant problem that is encountered when using a toner-transfer printing device for printing.
Jagged edges and certain other forms of image distortion can be addressed through a process called antialiasing, which involves smoothing or blurring the edges of high-contrast lines and curves. Antialiasing may be accomplished in a variety of ways. The result is an adjustment to the gray level of selected pixels to achieve a blurred effect: diagonal lines that otherwise would appear with jagged edges will have a blurred but smoother appearance.
Subsequent to an antialiasing operation, a toner-transfer printing device creates a printed image based on the adjusted pixel values by using continuous tone or halftone formats as appropriate to reproduce the image on the print medium. Although printing devices can selectively use continuous tone or halftone formats to generate each pixel on the print medium, current printing devices require a trade-off between the presence of artifacts and the presence of blurred edges. Although antialiasing techniques can enhance the perceived quality of an image, the blurred edges of an antialiased object fail to depict accurately the sharp features in the original image.
To print an image, a typical toner-transfer printing device creates an electrostatic representation of the pixels on a photoreceptive surface. In a typical laser printer, for example, a rotating drum with a photosensitive surface serves as the photoreceptive surface. Prior to forming a given printed image, the drum is cleaned and a uniform negative charge is applied to the drum's surface. The printing apparatus focuses laser light on selected portions of the drum's surface to create an electrostatic representation of the pixels. The laser light discharges the drum surface in the exposed regions. Portions of the drum surface that are not exposed to the laser light retain a negative potential, but the exposed portions have a higher potential. This process produces an electrostatic image on the drum's surface that is ready to be developed and transferred as a visible image to the print media.
To develop the electrostatic image into a visible image, developing material called toner is placed on the drum surface according to the electrostatic charge. Prior to being deposited on the drum's surface, the toner particles are given a negative charge; consequently, the toner particles are attracted to the areas of the drum surface that have acquired a higher potential through exposure to the laser light, and are repelled from the surface areas not exposed.
The next step is to transfer the toner from the drum to a print medium such as paper. A positive charge is applied to the back surface of the paper such that when the paper contacts the drum surface, the positive charges pull the negatively charged toner particles off the surface of the drum. The toner is fused to the paper by heat and pressure to produce a printed image.
The resolving power of a toner-transfer printing device is related to the granularity of the laser mechanism used to expose portions of the drum surface. A typical toner transfer device is capable of modulating the laser to expose an area significantly smaller than one pixel cell and thereby produce a discharged spot on the drum that is smaller than one pixel.
A technique referred to as pulse width modulation utilizes the high-resolution capability of toner-transfer printing devices to control the appearance of pixels. According to this technique, a printing device deposits toner only in a designated portion of a pixel by modulating the duration of the laser pulse used to expose a given pixel. The pulse-width modulation technique divides a pixel into a number of sub-elements and directs the printing device to expose only a specified number of sub-elements within a pixel, for example, several sub-elements on the left side of the pixel. A portion of a pixel created in this manner will hereinafter be referred to as a subpixel. By controlling the width of subpixels that are exposed to the laser light, the printing device can control the quantity of toner particles that adhere to the drum surface within each pixel cell.
Known halftoning techniques assign to a selected pixel a value representing a subpixel of a specified width. The width of the subpixel is commonly determined according to a number of parameters including the pixel's gray level. Known techniques, however, produce artifacts such as patterns of uneven subpixels along edges of an object. For purposes of this discussion, the term “object” means any distinct entity depicted in an image including text characters. “Background” means a portion of the image against which the object is displayed.
An additional problem associated with known pulse-width modulation techniques arises due to the printing device's use of electric charges during the printing process. As discussed above, toner particles are attracted to the areas of the photoreceptive material that have been exposed to laser light. Ordinarily, the quantity of toner particles attracted to an area of the material that corresponds to a pixel cell will be largely a function of the pixel's gray level; however, this electrostatic process can fail for an isolated subpixel that is surrounded by contrasting background pixels when the charge for the isolated subpixel is too weak to attract any toner particles. As a result, the printer fails to print the isolated subpixel. This phenomenon can be problematic when printing objects such as text. Under certain circumstances, known image processing techniques can cause subpixels at an edge of a halftoned object to be isolated from the pixels within the object. If the printing device fails to print the isolated subpixels, the object loses density and the object's appearance is degraded.
The problems outlined above relating to edge rendition and isolated subpixels can be especially severe for halftoned objects where a substantial portion of an object is represented by pixels that are not saturated. For purposes of this discussion, a pixel is saturated if it is represented by a subpixel that completely fills the area of the pixel cell. A pixel is not saturated if it is represented by a subpixel having a width that is less than the full width of the pixel cell.